Analysis and modelling of the seismic behaviour of high ... - Ingegneria

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2. DUCTILITY AND SEISMIC RESPONSE OF STRUCTURES 3. Survivability-ultimate limit state (ULS) for earthquakes which may rarely occur represents the strongest possible ground shaking. For these earthquakes, both structural and non-structural damage are expected, but the safety of inhabitants has to be guaranteed. In many cases damage is so substantial that structures are not repaired and demolition is the recommended solution. Although it is recognized that the ideal methodology would use four or three levels for design, today current European Code methodologies and seismic design philosophy can be based on just two levels (Eurocode 8, 2002): 1. Serviceability limit state, for which structures are designed to remain elastic, or with minor plastic deformations and the non-structural elements remain undamaged or have minor damage; 2. Ultimate limit state, for which structures exploit their capability to deform beyond the elastic range, the non-structural elements being partially or totally damaged. In the Eurocode 8, the accelerations corresponding to the serviceability limit state are given as a fraction of the corresponding ones for the ultimate limit state. Generally, this methodology cannot assure controlled damage, also because the determination of this relationship is not clearly assigned in the code. When the number of performance levels is discussed, one must recognize that the use of four levels is the most rational proposal and that two levels represent the minimum acceptable option. Since it is questionable to ask design engineers to perform too much verification, it seems more rational to introduce no more than three levels of verifications: serviceability, damageability and ultimate (survivability) levels. To achieve these levels of verifications, the seismic design problem is laid out through required-available formulation. Currently, the required-available pairs of three mechanical characteristics considered in seismic design are: Required rigidity < Available rigidity ( 2.3 ) Required strength < Available strength ( 2.4 ) Required ductility < Available ductility ( 2.5 ) The rigidity verification is a problem of the serviceability level, the strength verification is related to the damageability level and ductility verification is a 19
2. DUCTILITY AND SEISMIC RESPONSE OF STRUCTURES problem of survivability. Other verifications at each level are only optional or not necessary, as a function of building importance or earthquake intensities. The relationship between performance levels and the other parameters of structure- seismic design are: 20 • for serviceability level, under frequent low earthquakes, the strategy calls for the elastic response of a structure. The lateral deformations are limited for the interstorey drift limits, given for the non-structural elements. Due to their integrity, interaction structure-non-structural elements must be considered. The basic verification refers to the structure rigidity and the strength verification (condition to elastic behaviour) is only optional; • for damageability level, under occasional moderate earthquakes, an elasto-plastic analysis must be performed. The basic verification refers to the structure member strengths, verifications for rigidity or ductility being optional. The non-structural elements are partially damaged, so the analysis must consider only the structure behaviour, without any interaction with non-structural elements; • for ultimate (survivability) level, under rare severe earthquakes, a kinematic analysis, which considers the behaviour of possible formed plastic mechanisms, must be performed. The basic verification refers to the ductility, the strength verification being only optional. The design strategy refers to the control of the formation of a pre-selected plastic mechanism and the rotation capacities of plastic hinges. Non-structural elements are completely damaged. 2.4 Required ductility concept The required ductility is directly influenced by ground motions (source, distance from source, site conditions) and structural systems (foundations, structure types, non-structural elements). The control of ductility demand requires, firstly, attention to be given to some important aspects: i. Seismic macro-zonation, which is an official zoning map to country scale, based on a hazard analysis elaborated by geologists and seismologists. This map divides the national territory into different categories and provides each area with values of earthquake intensities, on the basis of design spectra. At the same time, this macro-zonation must characterize the possible ground motion types, as a surface or deep source an
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3. SEISMIC BEHAVIOUR OF BOLTED END
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5. SEISMIC BEHAVIOUR OF RC COLUMNS
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6. SUMMARY, CONCLUSIONS AND FUTURE
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Analysis and modelling of the seismic behaviour of high ... - Ingegneria

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